Image processing device

ABSTRACT

An image processing device for automatically setting the accumulation time (exposure time) in accordance with the illuminance even in a dark environment and smoothly following the motion of an object. The image processing device includes: gain control means ( 7 ) for performing gain control of a video signal from an imaging element ( 6 ) which focuses a signal from an iris ( 2 ) controlling the light quantity of the optical signal coming from outside and outputs a video signal; signal processing means ( 9 ) for signal-processing an output signal from the gain control means ( 7 ); and imaging control means ( 25 ) for controlling the opening degree of the iris ( 2 ), the exposure time of the imaging element ( 6 ), and the gain amount of the gain control means according to the video signal from the signal processing means ( 9 ). The imaging control means ( 25 ) judges the brightness around according to the video signal from the signal processing means ( 9 ) when it is dark around and changes the exposure time of the imaging element ( 6 ).

TECHNICAL FIELD

The present invention relates to an image processing device of a videocamera or the like capable of capturing a clear image in accordance withthe illuminance in a dark environment without depending on illuminationor the like.

BACKGROUND ART

A CCD type imaging element which is a solid imaging element is used formost of imaging elements for small video cameras or small video-integraltype cameras. Though a CMOS type imaging element is also used, it onlydiffers in the element of a photoelectrical conversion section and hasthe same process of obtaining an image signal. A case where a CCD typeimaging element is used as the above described solid imaging elementwill be explained as an example below.

In order to obtain a clear image in a dark environment, an image isnormally taken with enhanced illuminance of an object underillumination, but since lighting equipment is inconvenient to carry onand also involves great power consumption, it is preferable in the caseof a small video camera or the like that images be taken even in a darkenvironment without lighting equipment.

With regard to a camera using a solid imaging element, an example of adigital still camera is descried in Japanese Patent Laid-Open No.2001-285707. This document describes a system which automaticallycontrols exposure by using a CCD type imaging element, adjusting theimaging sensitivity, electronic shutter, gain of a CDS/AGC circuit whichamplifies signal output of the imaging element and values of controlparameters such as aperture value of an iris.

The present invention is a video camera which takes moving images andadjusts control parameters shown in Japanese Patent Laid-Open No.2001-285707 so as to set exposure in a dark environment, whereas fixedcontrol parameters are conventionally set with increased sensitivitywhen the environment is dark at a given moment. Especially, a long timeis set for the above described electronic shutter, that is, exposuretime. For this reason, there is a problem that the motion of the imagecaptured is not smooth.

The electronic shutter or so-called exposure time in a CCD type imagingelement is described in Japanese Patent Laid-Open No. 2001-285707, whichis constructed of the following system.

The CCD type imaging element has a light-receiving surface made up of aphotoelectrical conversion element which converts light of a photodiodearray (PD) or the like to electric charge, a accumulation section whichaccumulates charge generated and a charge transfer element (CCD) whichtransfers the charge in the accumulation section in vertical directionand in horizontal direction to obtain an image signal.

Thus, the input light quantity which is incorporated into the imagingelement is determined by the duration of accumulation of chargegenerated from the PD, and therefore if this duration of accumulation iscontrolled, it is possible to achieve temporal control over the lightquantity incorporated into the imaging element, that is, control of theexposure time without using any mechanical shutter. This is called an“electric shutter” or “electronic shutter.”

With a video camera using a CCD type imaging element, images are takenfor an electronic shutter time (exposure time) of 1/60 sec in accordancewith a field period (Tf) of a video signal in normal image taking. Thisis a case with image taking in a bright environment where there is asufficient input light quantity per unit time incorporated in 1/60 secand high signal output is obtained from the imaging element. In a darkenvironment, the input light quantity per unit time is small, andtherefore in order to increase a signal output, the input light quantityis accumulated with the exposure time extended so as to obtain a highsignal output.

Thus, high sensitivity image taking in a dark environment is realized byextending the period during which charge generated from the PD of theimaging element is accumulated in the accumulation section, which isequivalent to the electronic shutter time (exposure time).

For example, when the exposure time is set to 0.5 sec, this lengthcorresponds to 30 fields (15 frames), and therefore the charge from thePD for a period of 30 fields is accumulated in the accumulation section.The last 1 field of this accumulation period (exposure time) becomes anaccumulated image signal. Furthermore, the image signal of the last 1field of this accumulation period is signal-processed, converted to avideo signal and accumulated in memory for a period of 30 fields toobtain a continuous video signal. This exposure time can take any valueif it is a multiple of the frame period (approximately 33 ms) up toapproximately 0.5 sec, but it is often set to approximately 0.5 sec sothat images can be taken even in a considerably dark state.

However, in this case, the last 1 field out of the 30 fields(approximately 0.5 sec) accumulated is taken from the CCD, and so onestill image is obtained every 30 fields, and therefore the motion of theimage is not smooth. When the motion of the object is quick, imagetaking of the motion may be impossible. This may result in a problemthat it is not possible to obtain a video signal level corresponding tothe illuminance of the object.

The present invention is intended to solve the above described problemsand it is an object of the present invention to provide an imageprocessing device capable of automatically setting the above describedaccumulation time (exposure time) corresponding to illuminance at agiven moment even in a dark environment, following the motion of theobject as smoothly as possible and also optimizing the image quality atthat moment.

DISCLOSURE OF THE INVENTION

In order to solve these problems, the image processing device of thepresent invention is an image processing device provided with a firstimage-taking mode used when it is bright around and a secondimage-taking mode used when it is dark around, comprising an iris forcontrolling the light quantity of an optical signal coming from outside,an imaging element for outputting the optical signal from the iris as avideo signal, gain control means for performing gain control of thevideo signal from the imaging element, signal processing means forsignal-processing the output signal of the gain control means andimaging control means for controlling the opening degree of the iris,the exposure time of the imaging element and the gain amount of the gaincontrol means based on the video signal from the signal processingmeans, wherein the imaging control means judges the brightness around inthe second image-taking mode based on the video signal from the signalprocessing means and make changeable the exposure time in the imagingelement in accordance with the brightness.

Furthermore, the image processing device according to the presentinvention is an image processing device which enables image taking in adark environment by setting an electronic shutter-ON time which is anexposure time of an imaging element to an m·Tf (m: positive number)period within a period M·Tf (M: 1 and even number of 2 or greater, Tf:1-field period), comprising an imaging element made up of an imagingsurface consisting of photoelectrical conversion elements for convertinglight to charge, an accumulation section for accumulating the chargegenerated from the photoelectrical conversion element and a chargetransfer element (Charge-Coupled Device) for transferring theaccumulated charge in vertical and horizontal directions and obtainingan image signal, the imaging element consecutively changing the exposuretime m·Tf in a period M·Tf and automatically setting m·Tf to an optimumexposure time while maintaining a relationship: M·Tf=m·Tf+n·Tf, wheren·Tf (n: positive number of 0 to 2) is an electronic shutter-OFF time, alens unit made up of a lens for forming an object image on the imagingsurface of the imaging element and an iris or the like, an imagingelement driver which performs electronic shutter-ON drive control foraccumulating charge from the charge transfer element in the accumulationsection for the electronic shutter-ON time m·Tf, discharge drive controlfor discharging the charge from the accumulation section for theelectronic shutter-OFF time n·Tf and drive control for extracting animage signal of a last 1 field obtained for every the period M·Tfthrough vertical and horizontal transfers of the charge transfer elementaccumulated for the m·Tf time, an amplifier which amplifies the imagesignal obtained from the imaging element through driving of the imagingelement driver, a signal processing circuit which signal-processes theimage signal obtained from the amplifier to obtain a video signal madeup of a brightness signal and color signal, brightness detecting meansfor integrating the brightness signal indicating the light quantityvalue entering the imaging surface during the electronic shutter-ON timem·Tf for the last 1-field period of the exposure period and detectingthe input light quantity value corresponding to the brightness of theobject, brightness reference setting means for setting a reference valueof a brightness signal component corresponding to the brightness,comparison means for comparing a brightness signal component valueobtained from the brightness detecting means with the reference value ofthe brightness signal component from the brightness reference settingmeans and obtaining an error signal between both signals for everyperiod M-Tf and imaging element control means, wherein the imagingelement control means comprises exposure memory means for storing theelectronic shutter-ON time m·Tf set for every period M·Tf in memory,exposure time calculation means for subjecting an exposure timecorrection amount Δm−1·Tf obtained through a calculation based on theerror signal obtained 1 period ahead (M−1·Tf period) in a current period(MO-Tf period) during an electronic shutter-ON time m−1·Tf accumulatedin the exposure memory means 1 period ahead (M−1·Tf period) of thecurrent period (M0·Tf period) to addition or subtraction calculationprocessing according to the sign of the error signal and calculating anelectronic shutter-ON time m1·Tf (=m−1·Tf±Δm−1·Tf) in the next period(M1·Tf period) and control signal generating means for storing theelectronic shutter-ON time m1·Tf in the exposure memory means andgenerating a second control signal for extracting a 1-field image signalobtained by accumulating an electronic shutter-ON time supplied to theimaging element driver based on the electronic shutter-ON time m1·Tf andstoring a first control signal indicating an electronic shutter-OFFperiod, and first and second control signals generated based on theelectronic shutter-ON time m·Tf from the control signal generating meansare supplied to the imaging element driver, a feedback control loop isthereby formed during an M·Tf period, the electronic shutter-ON timem·Tf is changed and the electronic shutter-ON time (exposure time) m·Tfat a time point at which the error signal becomes zero or approximatesto zero is held to thereby obtain a video signal under an optimumexposure condition.

Furthermore, the present invention controls the above describedamplifier and iris so as to realize image taking in an environment ofall levels of brightness, covering a bright environment outside thebrightness range covered through control of the above described exposuretime and a dark environment outside the range to enable image taking inan environment of all levels of brightness.

This construction is a high-sensitivity image processing system capableof setting an optimum automatic exposure time using the function capableof accumulating the light quantity input to the imaging element of thevideo camera and can additionally achieve the following effects bycontrolling the iris and AGC amplifier:

(1) Setting an optimum exposure time in accordance with illuminancereduces deterioration of responsivity of motion of an output image whenthe exposure time is extended.

(2) A video signal output in accordance with the exposure time isobtained. That is, a video signal output in accordance with thebrightness is obtained.

(3) Image taking is possible in an environment ranging fromsubstantially complete dark to quite bright levels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an overall circuit according to Embodiment1 of the present invention;

FIG. 2 is a time chart illustrating the electronic shutter operation ofan imaging element according to the embodiment of the present invention;

FIG. 3 is a time chart illustrating the electronic shutter operation inthe case of a specific exposure time of the imaging element according tothe embodiment of the present invention;

FIG. 4 is a detailed circuit block diagram of imaging element controlmeans according to the embodiment of the present invention;

FIG. 5 is a detailed circuit block diagram of iris control meansaccording to the embodiment of the present invention;

FIG. 6 is a detailed circuit block diagram of AGC gain control meansaccording to the embodiment of the present invention;

FIG. 7 is a detailed circuit block diagram of selection signalgenerating means according to the embodiment of the present invention;

FIG. 8 is a time chart of input/output signals of selection signalgenerating means according to the embodiment of the present invention;

FIG. 9 illustrates a relationship between the brightness of an object,exposure time, between the brightness and iris value and between thebrightness and AGC gain value, and control areas according to theembodiment of the present invention; and

FIG. 10 illustrates a relationship between the brightness of an objectand brightness signal component value Y and basic signal reference valueYs according to the embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be explained with referenceFIGS. 1 to 10.

Embodiment 1

FIG. 1 shows an overall configuration of an embodiment of the presentinvention.

In FIG. 1, reference numeral 1 denotes a lens section for forming anobject image, 2 denotes an iris section made up of an iris whichmechanically adjusts an incoming light quantity and an iris drive motor(not shown) which changes the diameter of the iris and 100 denotes alens unit made up of the lens section 1 and iris section 2. Referencenumeral 20 denotes an iris mechanism driver which drives the iris drivemotor of the iris section 2. Reference numeral 19 denotes iris controlmeans for setting a diameter value of the iris.

Reference numeral 3 denotes a photoelectrical conversion element(referred to as “PD”) which converts light to charge such as aphotodiode array, the light quantity of which is adjusted by the lensunit 100 for photoelectrically converting the optical image formed.Reference numeral 4 denotes an array-shaped accumulation section whichaccumulates the charge from the PD 3 for a period during which theelectronic shutter is open, that is, exposure time (exposure period).Reference numeral 5 denotes a charge transfer element (hereinafterreferred to as “CCD”) which transfers the charge accumulated in theaccumulation section 4 in vertical and horizontal directions and obtainsan image signal. Reference numeral 6 denotes an imaging element made upof the PD 3, accumulation section 4 and CCD 5. Reference numeral 21denotes an imaging element driver which controls and drives the imagingelement 6 to extract an image signal from the imaging element 6.Reference numeral 18 denotes imaging element control means forgenerating timing signals to set the above described electronicshutter-ON time (exposure time) for the imaging element driver 21 andextract accumulated image signals.

An aplifier 7 is an amplifier made up of an AGC circuit and amplifiesthe image signal together with a CDS circuit which reduces noise of theimage signal obtained from the imaging element 6. Reference numeral 16denotes AGC gain control means for setting a gain of the AGC circuit ofthe amplifier 7.

An A/D converter 8 converts the image signal obtained from the aplifier7 to a digital signal. A signal processing circuit 9 converts thedigital image signal obtained from the A/D converter 8 to a digitalstandard video signal made up of a brightness signal and color signal.

Here, a normal image-taking mode (first image-taking mode) and ahigh-sensitivity image-taking mode (second image-taking mode) will beexplained.

The normal image-taking mode referred to here is a normal image takingstate within a range in which it is bright around, no illumination isrequired and an image captured can be normally judged. In this mode, asdescribed above, the electronic shutter time (exposure time) is set bythe imaging element control means 18 to 1/fv (fv: field frequency ofvideo signal) sec (approximately 1/60 sec) which is a 1/fv period of thevideo signal. Therefore, the field period (Tf) coincides with theelectronic shutter time (exposure time) in the normal image-taking mode,and therefore normal moving image taking can be performed. On the otherhand, in the high-sensitivity image-taking mode according to the presentinvention, effective image taking can be performed without illuminationeven in a situation in which a good image cannot be captured withoutillumination in the normal image-taking mode, and moreover, the devicein this mode can obtain a clear image signal even when the surroundingsituation changes and it becomes brighter. These modes can be switchedas follows.

A mode switching button 12 in FIG. 1 is a switching button whichgenerates a command signal for switching between the above described twomodes. Mode signal generating means 13 generates a control signal forchanging the setting condition of each control means and some means fromone mode to the other mode according to a command signal from the modeswitching button 12. When power to this apparatus is turned ON, thenormal image-taking mode is set. When the mode should be changed to thehigh-sensitivity image-taking mode, pressing the mode switching button12 allows the mode to be changed to the high-sensitivity image-takingmode by the above described command signal. Pressing the button againcauses the device to return to the original normal image-taking mode.This corresponds to a so-called toggle operation. Memory means 11 is amemory for storing a periodically obtained 1-field video signalaccumulated for an exposure time from the signal processing circuit 9forthat 1 period to convert the 1-field video signal to a continuous videosignal in the high-sensitivity image-taking mode. A switch means 22switches between the video signals obtained in both modes according to aswitching control signal from the mode signal generating means 13.

First, in the normal image-taking mode, as described above, a controlsignal is supplied from the mode signal generating means 13 to theimaging element control means 18 via a signal line 40 so that anelectronic shutter time which the imaging element control means 18 givesto the imaging element driver 21, that is, exposure time becomes abovedescribed 1/fv sec (=Tf). To make signal lines easily distinguishable,signal lines are numbered.

Therefore, a normal moving image image signal is obtained from theimaging element 6 as described above, subjected to amplification anddigital signal processing by the amplifier 7, A/D converter 8 and signalprocessing circuit 9respectively and a digital video signal ofconsecutive moving images is obtained from the signal processing circuit9. The digital video signal obtained is output via a terminal A of theswitch means 22. Reference numeral 23 denotes an output terminal andthis becomes an output terminal in the case of a specification of acamera only. Reference numeral 24 denotes a recorder which canrecord/reproduce a video signal such as digital video cassette recorderor disk recorder. In the case of an video-integral type camera, a videosignal of the switch means 22 is recorded by the recorder 24.

In the case of a normal image-taking mode, a setting of exposure timefrom the mode signal generating means 13 to the imaging element controlmeans 18 is 1/fv sec, while the AGC gain control means 16 which sets again of the AGC circuit of the aplifier 7 and the iris control means 19which sets an iris value a real so supplied with control signals forsettings from the mode signal generating means 13. The AGC gain is setto a minimum value (0 dB) and the iris is set to a steady-state value,but when the input light quantity increases, an iris value is setthrough iris control which will be described later.

This is the explanation of the operation in the normal image-takingmode.

The high-sensitivity image-taking mode according to the presentinvention applies to a device to obtain an acceptable image signal evenin a dark environment by setting an electronic shutter time, that is,exposure time, which is longer than a 1-field period (Tf=1/fv sec). Theoperation of the electronic shutter in this case will be explained usingFIG. 2 and FIG. 3.

In a video camera, an image consists of frame units each frame made upof odd and even fields of a video signal. Since the electronicshutter-ON time (hereinafter also referred to as “exposure time”)corresponds to a period during which charge from the PD 3 is accumulatedin the accumulation section 4, and therefore setting the exposure timeto a time exceeding a 1-field period requires periodic signal processingin units of several frames including the exposure time.

In the case of the high-sensitivity image-taking mode, the imagingelement driver 21 which drives and controls the accumulation section 4and CCD 5 of the imaging element 6 is supplied with control signalsshown in FIGS. 2(a), (b), (c) from the imaging element control means 18via signal lines 47, 46, 44. (FIGS. 2(a), (b), (c) correspond to (a),(b), (c) shown in signal lines 47, 46, 44 in FIG. 1).

A first control signal (a) in FIG. 2 is a signal for specifying anexposure time (charge accumulation period) and a discharge period and isa control signal indicating an exposure time Texp=m·Tf which correspondsto a period during which the charge from the PD 3 is accumulated in theaccumulation section 4 and an electronic shutter OFF-time Tdis=n·Tfwhich corresponds to a period during which the charge from the PD 3 isdischarged and no charge is accumulated in the accumulation section 4.Texp and Tdis are designed to have the following relationship so that anelectronic shutter operation is performed periodically. $\begin{matrix}\begin{matrix}{{{Tal}\quad 1} = {{Texp} + {Tdis}}} \\{= {{m \cdot {Tf}} + {n \cdot {Tf}}}} \\{= {\left( {m + n} \right){Tf}}} \\{= {M \cdot {Tf}}}\end{matrix} & (1) \\{{m + n} = M} & (2)\end{matrix}$where Tf=1-field period, m: a positive number of 1 to 34,n: positive number of 0 to 2, M: 1 or even number of 2 to 34or so and the relationship between m and M is expressed by the followingexpression:M=1 when m=1M=2 when 1<m≦2M−2<m≦M when 2<m   (3)

That is, an electronic shutter-ON/OFF operation is performed with M·Tfas 1 period which is the sum of m·Tf as an exposure time (electronicshutter-ON time) and n·Tf as a discharge period (electronic shutterOFF-time).

M=1 when m=1. This is the same as the exposure time (Tf=1/fv) in thenormal image-taking mode. FIG. 3(a) shows a control signal when m=2.5,that is, exposure time Texp=2.5Tf. In this case, since 2<2.5≦4 from theabove described relationship, M=4 and n=M−m=4-2.5=1.5, that is,Tdis=1.5Tf. Therefore, image taking is performed with the electronicshutter having an exposure time of 2.5Tf assuming 4Tf as 1 period.

Next, the charge accumulated in the accumulation section 4 during theexposure time m·Tf is extracted as an image signal through transfers bythe CCD 5 in the vertical and horizontal directions. For this purpose,the imaging element control means 18 supplies a signal indicating aperiod during which the accumulated charge shown in FIG. 2(b) istransferred to the CCD 5 or a charge transfer pulse indicating thevertical/horizontal transfer period and the image capturing period ofthe CCD 5 shown in FIG. 3(b) to the imaging element driver 21. Thispulse period corresponds to a vertical synchronization signal fly backperiod, during which charge is transferred from the accumulation section4 to the CCD 5. Furthermore, if the charge transferred to the CCD 5during a 1-field section for every M·Tf period is transferred in thevertical direction and horizontal direction by the CCD 5 itself using agate signal shown in FIG. 2(c) or FIG. 3(c), an image signal accumulatedfor the exposure time m·Tf for every M·Tf period is obtained from theCCD 5 of the imaging element 6. When this signal passes through theamplifier 7, A/D converter 8 and signal processing circuit 9, a 1-fieldvideo signal in which an optical image from the object is accumulatedfor an m·Tf period (exposure time) for every M·Tf period shown in FIG.2(d) or FIG. 3(d) as charge is obtained from the signal processingcircuit 9(FIG. 2(d) or FIG. 3(d) corresponds to a signal line 48(d) ofthe signal processing circuit 9 in FIG. 1).

In this case, the video signal from the signal processing circuit 9becomes a 1-field intermittent signal for every M·Tf period as describedabove, and therefore it is not possible to see the image withouttransforming it to a continuous video signal. Reference numeral 11denotes memory means for this purpose. The memory means 11 is suppliedwith the gate signal shown in FIG. 2(c) or FIG. 3(c) above via a signalline 44 from the imaging element control means 18 and the abovedescribed 1-field video signal is accumulated. When the period withoutany signal is replaced by the signal accumulated in the memory means 11,the continuous video signal shown in FIG. 2(e) or FIG. 3(e) is obtainedfrom the memory means 11 (FIG. 2(e) or FIG. 3(e) corresponds to a signalline 49(e) in FIG. 1).

In the high-sensitivity image-taking mode, a control signal from themode signal generating means 13 is supplied to the switch means 22 asdescribed above and a common terminal of the switch means 22 isconnected to a terminal B, and therefore the video signal shown in FIG.2(e) or FIG. 3(e) above from the memory means 11 is obtained at theoutput terminal 23 connected to the switch means 22 and in the case ofthe video-integral type camera, this signal is recorded by the abovedescribed recorder 24. This is the electronic shutter operation andvideo signal processing in the high-sensitivity image-taking mode.

Next, using this electronic shutter operation, the control methodcapable of automatically setting an optimum exposure time according tothe present invention will be explained below.

A 1-field intermittent video signal is obtained for every M·Tf periodshown in FIG. 2(d) or FIG. 3(d) above from the signal processing circuit9. As described above, object light is converted to charge by the PD 3,accumulated in the accumulation section 4, scanned by the CCD 5 and animage signal thereby obtained is subjected to signal processing at thesignal processing circuit 9 and converted to a video signal made up of abrightness signal component and color signal component. That is, sincethe brightness signal component is proportional to the light quantityfrom the object, an integral value for a 1-field section of thisbrightness signal component indicates the light quantity input to theimaging element 6 during an electronic shutter-ON period. Brightnessdetection means 10 integrates the brightness signal for 1 field toobtain the incoming light quantity during the electronic shutter-ONperiod m·Tf and detects a brightness signal component value Y(integration period corresponds to a section 113 shown in FIG. 2(c)).

Brightness reference setting means 14 includes a data table or the likestoring predetermined reference values of brightness signal componentvalues corresponding to the object illuminance and this data table isdetermined by an exposure time m·Tf value as will be described later.

Comparison means 15 compares the brightness signal component value Yfrom the brightness detection means 10 with a reference value Ys of thebrightness signal component from the brightness reference setting means14 and outputs an error signal Yd (=Y−Ys) thereof.

The error signal obtained by the comparison means 15 is supplied to theimaging element control means 18, gain control means 16 and iris controlmeans 19 via a signal line 39.

From above, a control loop is formed from imaging element 6→amplifier7→A/D converter 8→signal processing circuit 9→brightness detection means10→comparison means 15→imaging element control means 18→imaging elementdriver 21→imaging element 6. According to this control loop, the abovedescribed brightness signal component value equivalent to the chargeaccumulated for an exposure time m·Tf for every M·Tf period isperiodically compared with a reference value of the brightness signalcomponent for every period and feedback control is established so as todetermine the exposure time in the next period based on the error signalYd thereof.

As described above, since the error signal Yd=brightness signalcomponent Y - reference value Ys of brightness signal component, itwould be all right if the exposure time m·Tf can be automaticallyadjusted from these relationships such that the reference value Ys ofthe brightness signal component corresponding to the object illuminanceand brightness signal component Y for every M·Tf period. Therefore, inorder to calculate an exposure time when Y=Ys (not a complete matchingcondition but within a range of a certain width), when Y>Ys, the inputlight quantity is greater than the reference value (the objectilluminance is bright), that is, the current exposure time is long, andtherefore control can be performed such that the current exposure timebecomes shorter. On the contrary, when Y<Ys, the input light quantity issmaller than the reference value (the object illuminance is dark), thatis, the current exposure time is short, and therefore control can beperformed such that the current exposure time becomes longer using theabove described control loop. The means for calculating this exposuretime and generating a control signal is the imaging element controlmeans 18.

FIG. 4 is a specific block diagram of the imaging element control means18.

The error signal Yd is supplied to imaging element control means 18 fromthe comparison means 15 via the signal line 39. An exposure correctionvalue calculation means 31 is intended to calculate an exposure timecorrection value Δm·Tf for determining the exposure time in the nextperiod based on the error signal Yd and performs a calculation expressedby the following expression:Exposure time correction value Δm·Tf=error signal Ydx exposure timecorrection coefficient ks   (4)where ks is a constant. Since the light quantity is energy, Expression(4) can be expressed by a multiple-order function of Yd, yet it iscomplicated, and so it is expressed by a first-order expression.

Reference numeral 30 is first judging means for judging the sign of theerror signal Yd, judging the 0 (zero) value and generating a controlsignal. In other words:

error signal Yd=brightness signal component value Y—brightness signal Ys

and therefore the first judging means is the means for generatingrespective control signals by making the following decisions:

When Y>Ys, positive (+)

When Y=Ys, 0

When Y<Ys, negative (−)

As shown in FIG. 4, first switching means 32 changes the destination ofthe above described exposure time correction value Δm·Tf according to acontrol signal from the first judging means 30. Reference numeral 33denotes first subtraction means and 34 denotes first addition means.

Exposure time (electronic shutter-ON time) calculation processing means45 consists of the first judging means 30, first switching means 32,addition means 34 and subtraction means 33.

Exposure memory means 35 accumulates the value of the exposure time m.Tf obtained through a calculation by the exposure time calculation means45 and the value of the period M·Tf calculated from Expressions (1), (2)and (3) above based on this exposure time m·Tf until the next period.

As shown in FIG. 2, the exposure time m1·Tf in the next period iscalculated by the exposure time calculation means 45 in the currentperiod (M0·Tf period) and the exposure time m1·Tf (=exposure time m−1·Tfin the preceding period±exposure time correction value Δm−1·Tf) in thenext period is obtained. (The calculation period corresponds to thesection indicated by reference numeral 114 shown in FIG. 2(c)). In thisway, the value of the exposure time m−1·Tf in the preceding period(M−1·Tf) is delayed up to the current period (M0·Tf) by the exposurememory means 35, the exposure time m1·Tf and period M·Tf in the nextperiod calculated in the current period (M0·Tf) are obtained for everytwo periods and accumulated in memory. Reference numeral 38 denotescontrol signal generating means for generating the control signals shownin FIGS. 2(a), (b) and (c) given to the imaging element driver 21 fromthe values of the exposure time m·Tf and the period M·Tf for every twoperiods obtained from the exposure memory means 35.

As described above, when Y>Ys, a positive control signal is suppliedfrom the first judging means 30 through the first switching means 32,and therefore the above described exposure time correction value Δm·Tfis supplied to a subtraction (−) input of the subtraction means 33 via aterminal b(+). The value of an exposure time m−1·Tf of the currentperiod M0·Tf in the preceding period M−1·Tf shown in FIG. 2(a) issupplied to an addition (+) input of the subtraction means 33 from theexposure memory means 35 and an exposure time corresponding to the nextperiod expressed by the following Expression is obtained from thesubtraction means 33.m1·Tf=m−1·Tf−Δm−1·Tf   (5)

Y>Ys means that the brightness signal component value obtained by theexposure time m−1·Tf in the preceding period M−1·Tf is greater than areference value, that is, the exposure time in the preceding period islong, and therefore if the next period is shortened, Y approximates toY=Ys.

The exposure time m1·Tf in the next period calculated by Expression (5)is shorter than the exposure time m−1·Tf in the preceding period by theexposure time correction value Δm−1·Tf in the preceding periodcalculated by Expression (4) above. These relationships are also shownin FIG. 4.

On the other hand, when Y<Ys, the first switching means is changed to aterminal a (−), and therefore Δm·Tf is supplied to one addition input ofthe addition means 34. The value of the exposure time m−1·Tf in theabove described preceding period M−1·Tf is supplied to the otheraddition input and the exposure time corresponding to the next periodexpressed by the following expression is obtained from the additionmeans 34.m1·Tf=m−1·Tf+Δm−1·Tf   (6)

Y<Ys means that the brightness signal component value obtained for theexposure time m−1·Tf in the preceding period M−1·Tf is smaller than thereference value, that is, the exposure time in the preceding period isshort, and therefore if the exposure time in the next period isextended, Y approximates to Y=Ys.

The exposure time m1·Tf in the next period calculated according toExpression (6) is longer than the exposure time m−1·Tf in the precedingperiod by the exposure time correction value Δm−1·Tf in the precedingperiod calculated according to Expression (4) above. These relationshipsare also shown in FIG. 4.

FIG. 9 is a graph showing the above described control system in arelationship between the brightness of an object and exposure time. Thehorizontal axis shows the brightness (illuminance) of the object. Thebrightness detected here is an incoming light quantity and shows from astate in which the iris is maximum, that is, opened to the full, thebrightest (position of dotted line 130) to a dark state (position ofdotted line 142). The vertical axis shows the exposure time m·Tf andperiod M·Tf to be set corresponding to the brightness of the object andalso shows an iris value I and AGC gain value G in the iris control. Asshown in the figure, there are four control areas according to thebrightness. ALC 120 has the same range as that in the above describednormal image-taking mode and the exposure time is fixed to a 1-fieldperiod length 1 Tf (1/fv) as indicated by reference numeral 124 a. Onlythe iris is controlled. The iris value I is expressed by an aperturediameter. If this is expressed with an F value, the stop is closed (theaperture diameter is a minimum value) when it is brightest, andtherefore the F value is max. As the brightness of the object becomesdarker from that state, the stop is opened (the aperture diameterincreases and the F value decreases) and control is performed such thatan iris value corresponding to the brightness is set until the irisvalue I becomes Ist (Fr.s in F value). The range of this ALC 120 iscontrolled by the iris control means 19. STC indicated by referencenumeral 121 denotes a control area in which an optimum exposure timem·Tf corresponding to the brightness of the object by the abovedescribed imaging element control means 18 is set. The relationshipbetween the brightness and the exposure time m·Tf is as shown by thecurve indicated by reference numeral 124 b. From the relationship inExpression (3), the period M·Tf is 2Tf when the exposure time m·Tfranges from 1Tf to 2Tf as indicated by reference numeral 160 a and 4Tfwhen m·Tf ranges from 2Tf to 4Tf as indicated by reference numeral 160b. Thus, M·Tf has a stepped shape incrementing in 2Tf units according tothe value of m·Tf as shown in the figure. In this STC area, the irisvalue I is fixed to standard Ist (Fr.s in F value). IRIS indicated byreference numeral 122 is a control area by the iris and controlled bythe iris control means 19. AGC indicated by reference numeral 123 is acontrol area in the darkest range and controlled by the gain controlmeans 16. In the areas of the IRIS 122 and AGC 123, the exposure time isfixed to a maximum value (34Tf shown in the figure). That is, therelationship between the brightness and electronic shutter-ON time(exposure time) of the object is fixed to a 1-field period length 1Tf(1/fv) in the area of ALC 120. In the area of the STC 121 thereafter,m·Tf changes as shown by the curve indicated by reference numeral 124 baccording to the brightness and the exposure time is set in accordancewith the brightness through the above described control of the imagingelement control means 18. In the areas of the IRIS 122 and AGC 123, m·Tfis fixed to a maximum value (=34Tf) as indicated by reference numeral124 c.

The relationship between the brightness and iris value in the area ofALC 120 changes rectilinearly from Imin (F value is Fmax) to Ist (Fr.s)as shown by the solid line indicated by reference numeral 125 a and isset to an I value corresponding to the brightness. In the area of STC121, it is fixed to standard Ist (Fr.s) as shown by the solid lineindicated by reference numeral 125 b. That area can be said to be thearea for setting the exposure time m·Tf corresponding to the brightnessas described above under that condition. The F value of the area of IRIS122 changes rectili nearly from Ist (Fr.s) to Imax (Fmin) (open) asshown by the solid line indicated by reference numeral 125 c and is setto an iris value corresponding to the brightness in that range. The AGC123 is fixed to Imax (Fmin) as shown by the solid line indicated byreference numeral 125 d. Curves 125 a, 125 b, 125 c and 125 d showingthe relationship between the brightness and iris value I are expressedby the relationship between the brightness and iris aperture diameter,which is opposite to the relationship of the F value. As shown in thefigure, when it is bright, the aperture diameter is reduced and theaperture diameter is reduced to a minimum value Imin in the brightestcondition, while the F value is maximum Fmax. On the contrary, theaperture diameter reaches a maximum value Imax in the darkest conditionand the F value reaches a minimum value Fmin.

The relationship between the brightness and AGC gain, which is anothercontrol parameter is fixed to a min value (=0 dB) in the areas of ALC120, STC 121 and IRIS 122. This means that an output is obtained fromthe aplifier 7 even when the gain of the AGC circuit is 0 dB in theseranges. The range of AGC 123 is a range within which no output isobtained unless the AGC gain is increased within a considerably darkrange, which varies as shown by a dotted line 126 and control isperformed such that it is set to an AGC gain value corresponding to thebrightness in the range.

Next, the above described brightness signal component value Y, referencevalue Ys of the brightness signal component and difference signal (errorsignal) Yd between them will be explained using FIG. 10. In FIG. 10, thesame lines and ranges as those in FIG. 9 are assigned the same referencenumerals. The horizontal axis in FIG. 10 is the axis showing thebrightness as in the case of FIG. 9. The vertical axis shows abrightness component value Y and reference value Ys of the brightnesssignal component. As in the case of FIG. 9, the horizontal axis showsareas from ALC 120 to AGC 123. In the respective areas, the referencevalues Ys of the brightness signal component in the respective areas areaccumulated in a data table of a non-volatile memory or the like in thebrightness reference setting means 14 as predetermined values. In thearea of ALC 120, an incoming light quantity enough to take a maximumvalue of the brightness signal output can be obtained, and therefore ifan iris value corresponding to the brightness so that Y reaches amaximum value Yh(=Ys) shown by reference numeral 127 a can be set, it ispossible to realize image taking under an optimum condition. In the areaof STC 121, since the brightness of the object gradually becomes darker,the reference value Ys of the brightness signal component is set to avalue that matches the brightness as shown by reference numeral 127 b.As explained in FIG. 9, an exposure time corresponding to the brightnessis set in the area of STC 121, and therefore the exposure time and thebrightness have a one-to-one correspondence. Positions indicated byreference numerals 131 to 140 in FIG. 9 show representative exposuretimes. FIG. 10 also shows representative values (dotted line 131 todotted line 140) of the exposure time corresponding to the brightness.From this, in the area of STC 121, the reference value Ys of thebrightness signal component may be defined as a function of the exposuretime m·Tf.Ys=F(m·Tf)   (7)In the areas of IRIS 122 and AGC 123, the reference value Ys of thebrightness signal component is set to Y1 which prevents noise fromincreasing. In any way, the reference value Ys of the brightness signalcomponent is determined by the exposure time in all areas. That is,

Area of ALC: Ys=F(1 Tf)=Yh

Area of STC: Ys=. F(m·Tf)

In the areas of IRIS and AGC, Ys=F(34 Tf)=Y1. In all areas, thereference value Ys of the brightness signal component can also be set toYs=Yh as indicated by a dotted line 128, but bright screens are obtainedin all areas, which is not practical.

The relationship between Ys, Y and Yd and the exposure time based onExpression (5) above obtained through a calculation at the exposure timecalculation processing 45 will be explained using FIG. 10. Suppose thatthe exposure time in the preceding period M-l-Tf (see FIG. 2(a)) is 26.5Tf shown by a dotted line 143 . M−1·Tf at that time is 28 Tf fromExpressions (1), (2) and (3). In this preceding period, the chargeaccumulated for an exposure time 26.5 Tf is converted to a 1-field videosignal in the current period MO-Tf (see FIG. 2(a)), and it is Y thatintegrates only the brightness signal component of this signal. Thepoint indicated by reference numeral 129 a is the value of this Y.Suppose this is Ya. The point indicated by reference numeral 129 bbecomes Ys corresponding to the exposure time 26.5 Tf. Assuming that Ysat this time is Ysb, since the error signal Yd is the difference betweenY and Ys, Yd is expressed by a solid line 144 between arrows andexpressed by the following expression:Yd=Y−Ys=Ya−YsbAn exposure time correction value Δm·Tf expressed by Expression (4) isobtained by the exposure correction value calculation means 31.$\begin{matrix}{{\Delta\quad{m \cdot {Tf}}} = {{{Yd} \cdot {ks}} = {{\left( {{Ya} - {Ysb}} \right) \cdot {ks}} = {{\Delta\quad m} - {1 \cdot {Tf}}}}}} & (8)\end{matrix}$This value can be approximately estimated from FIG. 10. Ys having thesame value as Ya is a point 129 c, which is the intersection between thecurve shown by a dotted line 145 (this dotted line is the solid line 127b turned upside down and passes through the point at 129 a) and thecurve shown by reference numeral 127 b. Suppose this point Ys is Ysc. Adotted line 146 which passes through this point has an exposure time of14.5 Tf (close to 14 Tf indicated by a dotted line 136). From these:Δm−1·Tf=26.5Tf−14.5Tf=12In practice, this is calculated according to Expression (8). Δm−1·Tf isobtained from a correlation by multiplying the difference between Y andYs by a correction coefficient ks. The exposure time m1·Tf in the nextperiod M1·Tf is calculated from Expression (5) as: $\begin{matrix}{{m\quad{1 \cdot {Tf}}} = {m - {1 \cdot {Tf}} - {\Delta\quad m} - {1 \cdot {Tf}}}} \\{= {{26.5{Tf}} - {12{Tf}}}} \\{= {14.5{Tf}}}\end{matrix}$In the next period, Y substantially matches Ys, and therefore if it isaccumulated in the exposure memory means 35 for that exposure time, 14.5Tf in this case, it is possible to realize image taking under anexposure condition that matches the brightness. Detecting a matchbetween Y and Ys equals detecting that Yd is 0. This detection isperformed by the first judging means 30. If Yd=0, that is, Y=Ys=Ysc, acontrol signal is supplied from the first judging means 30 to theexposure memory means 35, and in subsequent periods, the exposure timeaccumulated at that time point is held. The area of STC 121 iscontrolled as shown above. Next, control over the area of ALC 120 havinga brighter object illuminance than the area of STC 121 will beexplained.

The control in this area is performed by the iris control means 19 inFIG. 1. The exposure time in this area is 1-field period 1 Tf (=1/fv) asdescribed above. FIG. 5 is a detailed block diagram of the iris controlmeans 19. An error signal Yd (=Y−Ys) from the comparison means 15 issupplied to the iris control means 19 via a signal line 39. Iris valuecalculation means 66 calculates an iris value in the next period duringthe current period based on the error signal Yd in the preceding period.Reference numeral 55 denotes iris value memory means for storing irisvalues in the preceding period, current period and next period inmemory, supplying the iris values to the iris mechanism driver 20 andgenerating a control signal for setting the iris 2.

The calculation processing in the iris value calculation means 66 willbe performed as follows. Reference numeral 50 denotes iris correctionvalue calculation means for performing a calculation based on the errorsignal Yd in the preceding period as shown in the following expression:ΔI=Yd·ki   (9)where, ΔI: iris correction value, ki: iris correction coefficient(constant).

Second judging means 52 judges the sign of the error signal Yd andjudges the value 0 (zero) and generates a control signal. Since:

error signal Yd=brightness signal component value Y−reference value ofbrightness signal component Ys and therefore this is the means forgenerating respective control signals by making the following decisions:

When Y>Ys, positive (+)

When Y=Ys, 0

When Y<Ys, negative (−)

Reference numeral 51 denotes second switching means for switching thedestination of the above described iris correction value ΔI and isswitched according to a control signal from the second judging means 52as shown in FIG. 5. Reference numeral 53 is second subtraction means andreference numeral 54 denotes second addition means.

In this case, the brightness signal component value Y in the precedingperiod (field) is detected and compared with the reference value Ys ofthe brightness signal component (in this area, Ys=Yh (constant) as shownin FIG. 10). Based on the error signal Yd, an iris correction value ΔI−1in the preceding period (field) shown by Expression (9) is calculatedusing the iris correction means 50. In the current period (field),depending on whether the Yd is positive or negative, an addition orsubtraction is performed between the iris value I−1 in the precedingperiod (field) obtained from the iris value memory means 55 and abovedescribed iris correction value ΔI−1 by the second addition means 54 orsecond subtraction means 53 and an iris value I1 for the next period iscalculated. 2-field cycle control is performed in such a way that theiris value I1 obtained is executed in the next period and an iris valueI when Y=Ys(Yh), that is, Yd=0 is held. Yd=0 is judged by the secondjudging means 52. When Yd=0, a control signal is supplied from thesecond judging means 52 to the iris value memory means 55 and the irisvalue at that time point is accumulated in memory and held, andtherefore an optimum iris value corresponding to the brightness is setand optimum image taking can be realized. This is the method ofcontrolling the area of ALC 120. Next, control over the area of IRIS 122having an object illuminance darker than the area of STC 121 will beexplained.

This area is controlled by the iris control means 19 in the same way asthe area of ALC 120. The operation of the iris control means 19 in thisarea is basically the same as the above described area of ALC 120, butit differs in the control cycle, that is, exposure time m·Tf (=periodM·Tf) and reference value Ys of the brightness signal component. Acomparison is shown below (see FIG. 9 and FIG. 10). Reference value Ysof M · Tf (=period M · Tf) brightness signal component ALC 1Tf(constant) Yh IRIS Maximum value (34Tf) Yl (constant)

In the ALC area, the period of the exposure time is 1 field (1 Tf)cycle. The actual control cycle is controlled in a 2 Tf cycle asdescribed in the control of the ALC area. The period in the IRIS area isa 34 Tf cycle and this corresponds to approximately a 0.56-sec cycle.The control cycle is 68 Tf, double this cycle. Therefore, it is a1.1-sec cycle. This area exists to enable image taking in a considerablydark condition, and therefore the aperture diameter of the iris isincreased and control is performed so as to set the iris correspondingto the object illuminance in order to set the exposure time to a maximumvalue and further increase the sensitivity. The method of controllingthe iris control means 19 is only different in the Ys from the abovedescribed cycle and has the same circuit operation as that in the ALCarea, and therefore explanations thereof will be omitted. This is thecontrol method for the IRIS area.

Next, the area of AGC 123 whereby an area darker than the area of IRIS122 is controlled will be explained. As shown in FIG. 9 and FIG. 10, theexposure time is also a maximum in this area and control is performed soas to further increase the sensitivity with a maximum (F value minimum)of the aperture diameter of the iris, that is, in an OPEN state andenable image taking in a dark state. The control over this area isperformed by the AGC gain control means 16. The period of the exposuretime in this area is 34 Tf cycle as described above. FIG. 6 is adetailed circuit block diagram of the AGC gain control means 16. Theerror signal Yd (=Y−Ys) from the comparison means 15 is supplied to theAGC gain control means 16 via the signal line 39. Reference numeral 78is gain calculation means for calculating a gain value in the nextperiod during the current period based on the error signal Yd in thepreceding period. Reference numeral 75 is AGC gain value memory meansfor storing gain values in the preceding period, current period and nextperiod in memory, supplying the gain values to the amplifier includingthe AGC circuit and generating a control signal for setting the gain ofthe AGC circuit.

The calculation processing by the gain calculation means 78 will beperformed as follows. Reference numeral 70 is AGC gain correction valuecalculation means for performing a calculation shown in the followingexpression based on the error signal Yd in the preceding period.ΔG=Yd·kg   (10)where ΔG: gain correction value, kg: gain correction coefficient(constant).

Third judging means 72 judges the sign of the error signal Yd, judgesthe value of 0 (zero) and generates a control signal. The error signalYd=brightness signal component value Y—reference value of brightnesssignal component Ys, and therefore this is the means for generatingrespective control signals by making the following decisions:

When Y>Ys, positive (+)

When Y=Ys, 0

When Y<Ys, negative (−)

Reference numeral 71 denotes third switching means for switching thedestination of the above described gain correction value ΔG and isswitched according to a control signal from the third judging means 72as shown in FIG. 6. Reference numeral 73 is third subtraction means andreference numeral 74 denotes third addition means.

In this case, the brightness signal component value Y corresponding tothe input light quantity accumulated in the preceding period is detectedand compared with the reference value Ys of the brightness signalcomponent (in this area, Ys=Y1 (constant) as shown in FIG. 10). Based onthe error signal Yd, the gain correction value means 70 calculates again correction value ΔG−1 in the preceding period shown by Expression(10). In the current field, depending on whether the Yd is positive ornegative, an addition or subtraction is performed between the gain valueG−1 in the preceding period obtained from the AGC gain value memorymeans 75 and above described gain correction value ΔG−1 by the thirdaddition means 74 or third subtraction means 73 and a gain value G1 forthe next period is calculated. 2-field cycle control is performed insuch a way that the gain value G1 obtained is executed in the nextperiod and a gain value G when Y=Ys (Y1), that is, Yd=0 is held. Yd=0 isjudged by the third judging means 72. When Yd=0, a control signal issupplied from the third judging means 72 to the gain value memory means75 and the gain value at that time point is accumulated in memory andheld, and therefore an optimum gain value corresponding to thebrightness is set and optimum image taking can be realized. This is thecontrol method in the AGC area.

In this way, the control over four areas has been explained individuallyand it is an object of the present invention to set an optimum exposuretime, iris value and AGC gain value in accordance with the brightness ofthe object in order to realize effective image taking in a darkenvironment. That is, when a situation is changed from a state in whichimage taking is performed in a normal image-taking mode to image takingin a dark environment, it is an object of the present invention tochange the mode to the above described high-sensitivity image-takingmode, obtain the exposure time, iris value, AGC gain value (hereinafterreferred to as “3 optimum set values”) under an optimum condition whichmatches the brightness in the above described four areas and maintaintheir values. For that purpose, a method of obtaining three optimum setvalues which match the brightness so as to automatically sweep the abovedescribed four areas will be explained.

The selection signal generating means 17 in FIG. 1 generates controlsignals to switch the imaging element control means 18, iris controlmeans 19 and AGC gain control means in order to automatically sweep theabove described four areas. FIG. 7 is a block diagram thereof and FIG. 8shows time charts of signals on the respective signal lines.

Reference numerals 93, 95, 96 in FIG. 7 denote OR gates, 94, 97 denoteflip flops, 98 denotes a NOR gate. The operation of the selection signalgenerating means 17 including these circuits will be explained below.

First, when the image taking condition is changed from a normalimage-taking mode to a high-sensitivity image-taking mode (the modeswitching button 12 is pressed for this switching as described above),the mode signal generating means 13 supplies a start signal shown inFIG. 8(a) to the selection signal generating means 17 via a signal line99. At the same time, the mode signal generating means 13 suppliesinitial values at the control start to the exposure memory means 35 inthe imaging element control means 18, iris value memory means 55 in theiris control means 19 and AGC gain value memory means 75 in the AGC gaincontrol means 16 respectively. These initial values are prestored in thedata table or the like in the mode signal generating means 13.

As shown at the control start point in FIG. 9, a maximum value (34 Tf)is supplied to the exposure memory means 35 as the initial value of theexposure time, Imax (Fmin) is supplied to the iris value memory means 55and a maximum value (Gmax) is supplied to the AGC gain value memorymeans 75 and stored in the respective memories. This is done because itis unknown in which area of the above described four areas thebrightness of the object is located, and so the control is started fromthe darkest state when the image-taking mode is switched to thehigh-sensitivity image-taking mode. This start signal passes through theOR gate 93 and is supplied to S (set input) of the flip flop 94.Therefore, a control signal G is obtained at the output Q of the flipflop 94 as indicated in FIG. 8(h) which rises the moment the startsignal enters. This control signal G is supplied to the AGC gain controlmeans 16 via a signal line 92. This control signal G is supplied to theAGC gain value memory means 75 and AGC gain correction value calculationmeans 70 in the AGC gain control means 16. These means operate for aperiod during which this control signal G is at H level, the gaincorrection value as the output of the AGC gain correction valuecalculation means 70 is held to a 0 (zero) value for a period duringwhich the control signal G is at L level and the AGC gain value memorymeans 75 holds the last memory value (minimum value) at the end of theoperation. That is, the AGC gain value memory means 75 holds the memoryvalue at the time that the level of the control signal G changes from Hto L.

Thus, control is started assuming a dark condition as shown in FIG. 9,FIG. 10 and the above described three optimum set values correspondingto the above described brightness in the AGC area are determined by theAGC gain control means 16. If the brightness of the object is located inany part of the AGC area, there is a time point at which the value ofthe above described error signal Yd becomes 0, an AGC gain value Gx atthat time point is stored and held in the AGC gain value memory means75. That is, the exposure times of the three optimum set values at thistime point are maximum values (34 Tf), the iris has a maximum value Imax(OPEN), the AGC gain becomes Gx and the imaging element 6, iris 2 andaplifier 7 operates with these values. While these values are held, theabove described control signal G (see FIG. 8(h)) is held at H level.This means that the AGC gain control means 16 is operating (the setvalue is determined at any point in a section A of the time chart shownin FIG. 8).

Next, when-the brightness of the object is in the area of IRIS, controlis started by pressing the mode switching button 12 and the start signaland initial value or the like are set as described above, but in thearea of AGC, the error signal Yd (=Y−Ys) obtained from the comparisonmeans 15 is Yd>0, that is, Y>Ys, and therefore this control area needsto be surpassed and changed to the next control area of IRIS. Theturning point at which the area of AGC is switched to the area of IRIScan be found by detecting a time point at which the AGC gain valuebecomes a minimum value (0 dB) as shown in FIG. 9. Reference numeral 76in FIG. 6 denotes minimum gain judging means, which generates a controlsignal for surpassing the area of AGC and entering the control area ofthe area of IRIS. According to the control method in the AGC area,control is performed such that an optimum value of the AGC gain value iscalculated in the period of the exposure time maximum value (34 Tf) asdescribed above, and therefore control is performed in a direction inwhich the gain value is reduced. Even when the gain is reduced, Y>Ys,and therefore after several control cycles, a time point appears atwhich the AGC gain value from the third subtraction means 73 in the AGCgain control means 16 becomes a minimum value. The minimum gain judgingmeans 76 detects the time point at which the minimum value is reachedand generates a gain minimum value arrival signal as shown in FIG. 8(b).This signal is supplied to a reset input R of the flip flop 94 and ORgate 95 of the selection signal generating means 17 shown in FIG. 7 viaa signal line 80. Therefore, the flip flop 94 is reset and the controlsignal G shown in FIG. 8(h) is obtained at the output Q, and when thissignal becomes L level, the control by the AGC gain control means 16 isstopped and as described above, the gain minimum value stored in the AGCgain value memory means 75 is supplied to the amplifier 7 including theAGC circuit from this time point onward. This gain minimum value arrivalsignal is passed through the OR gate 95 and also supplied to the setinput S of the flip flop 97, and therefore a control signal I shown inFIG. 8(h) is obtained at the output Q of the flip flop 97. This controlsignal I is supplied to the iris value memory means 55 and iriscorrection value calculation means 50 in the iris control means 19. Fora period during which this control signal I is at H level, these meansoperate, the gain correction value as the output of the iris correctionvalue calculation means 50 is held to a zero value for a period duringwhich this control signal I is at L level and the AGC gain value memorymeans 55 holds the final memory value (minimum value) at the end of theoperation. That is, the AGC gain value memory means 55 holds the memoryvalue at the time that the control signal I changes from the H level toL level. In this way, the period during which the control signal I is atH level corresponds to the period during which the iris control means 19is operating.

During this operation period, if the brightness of the object is at someposition in the IRIS area, there is a time point at which the value ofthe above described error signal Yd becomes 0 and an iris value Ix atthat time point is stored and held by the iris value memory means 55.That is, for the three optimum set values at this time point, theexposure time becomes a maximum value (34 Tf), the iris is Ix, the AGCgain becomes a minimum value (0 dB) and the imaging element 6, iris 2and aplifier 7 operate with these values.

While these values are held, the above described control signal I (seeFIG. 8(i)) is held at H level, which means that the iris control means19 is operating (the set value is determined at some position in sectionB in the time chart shown in FIG. 8).

Next, when the brightness of the object is in the STC area (see FIG. 9and FIG. 10) 121, the control is started by pressing the mode switchingbutton 12, the start signal and initial value are set as shown above,but since the error signal Yd (=Y−Ys) obtained from the comparison means15 is Yd>0, that is, Y>Ys in the AGC area 123 and IRIS area 122, andtherefore these areas are passed. At a turning point at which the AGCarea 123 is surpassed and the IRIS area 122 is switched to the STC area121, a point at which the iris value becomes Ist (Fr.s) (point b) asshown in FIG. 9 may be detected. The first iris value judging means 57in FIG. 5 is intended to detect a point at which the iris value becomesIst (Fr.s) (point b).

According to the control method in the IRIS area 122, control isperformed such that an optimum value of the iris value is determined ina period corresponding to an exposure time maximum value (34 Tf) asdescribed above, and therefore the control is performed in a directionin which the iris value is reduced. Even when the iris value is reduced,Y>Ys, and therefore after several control cycles, a time point appearsat which the iris value from the second subtraction means 53 at the iriscontrol means 19 becomes Ist (Fr. s). The first iris value judging means57 detects a time point at which the iris value becomes Ist(Fr.s) andgenerates an iris value Ist (point b) arrival signal as shown in FIG.8(c). This arrival signal is supplied to the OR gate 96 of the selectionsignal generating means 17 shown in FIG. 7 via the signal line 64.Furthermore, this arrival signal is passed through the OR gate 96 andsupplied to the reset input R of the flip flop 97, and therefore theflip flop 97 is reset and the control signal I shown in FIG. 8(i) isobtained at the output Q. When this signal becomes L level, the controlby the iris control means 19 is stopped and as described above, the irisvalue Ist (Fr.s) stored in the iris value memory means 55 is supplied tothe iris mechanism driver 20 from this time point onward. On the otherhand, the control signal G which is the output Q of the flip flop 94shown in FIGS. 8(h) and (i) and control signal I which is the output Qof the flip flop 97 are supplied to the NOR gate 98 in the selectionsignal generating means 17. Therefore, a control signal P shown in FIG.8(j) is obtained at the output of the NOR gate 98. This control signal Pis supplied to the exposure memory means 35 and exposure correctionvalue calculation means 31 in the imaging element control means 18 via asignal line 90. These means operate for a period during which thiscontrol signal P is at H level, the exposure time correction value asthe output of the exposure correction value calculation means 31 is heldto a zero value for an L level period and the exposure memory means 35holds the last memory value (minimum value) at the end of the operation.That is, the exposure memory means 35 holds the memory value at a timepoint at which the control signal P is changed from H level to L level.The period during which the control signal P is at H level correspondsto a period during which the imaging element control means 18 isoperating.

During this operation period, if the brightness of the object is at someposition in the STC area, there is a time point at which the value ofthe above described error signal Yd becomes zero and the exposure timemxTf at that time point is stored and held in the exposure memory means35. That is, for the three optimum set values at this time point, theexposure time is mxTf, the iris value is Ist (Fr. s), the AGC gainbecomes a minimum value (0 dB) and the imaging element 6, iris 2 andaplifier 7 operate with these values. While these values are held, theabove described control signal P (see FIG. 8(j)) is held at H level.This means that the imaging element control means 18 is operating (theset value is determined at some position in a section C of the timechart shown in FIG. 8).

Next, when the brightness of the object is in the ALC area (see FIG. 9and FIG. 10) 120, the control is started by pressing the mode switchingbutton 12, the start signal and initial value are set as shown above,but since the error signal Yd (=Y−Ys) obtained from the comparison means15 is Yd>0, that is, Y>Ys in the AGC area 123, IRIS area 122 and STCarea 121, and therefore these areas are passed through the control ofthe control areas. At a turning point at which the AGC area 123 and IRISarea 122 are surpassed and the STC area 121 is switched to the ALCarea-120, a point at which the exposure time becomes a minimum value (1Tf) may be detected as shown in FIG. 9. The minimum exposure judgingmeans 36 in FIG. 4 is the exposure time minimum value (1 Tf) judgingmeans for generating a control signal for surpassing the STC area 121and switching to the control area of the ALC area 120. According to thecontrol method in the STC area 121, control is performed such that anoptimum value of the exposure time is determined by changing theexposure time from the exposure time maximum value (34 Tf) to theminimum value (1 Tf) as described above, and therefore the control isperformed in a direction in which the exposure time is reduced in thebright direction. When the brightness is in the ALC area 120, Y>Ys inthe state of control of the STC area 121, that is, state of control bythe imaging element control means 18, and therefore after severalcontrol cycles, a time point appears at which the exposure time obtainedfrom the first subtraction means 33 of the imaging element control means18 becomes a minimum value (1 Tf). The minimum exposure judging means 36detects the time point at which the exposure time becomes the minimumvalue (1 Tf) and generates an exposure time minimum value (1 Tf) arrivalsignal as shown in FIG. 8(d). This arrival signal is supplied to the ORgate 95 of the (control means selection control signal generating means)selection signal generating means 17 shown in FIG. 7 via a signal line42. Furthermore, this arrival signal is passed through the OR gate 95and supplied to the set input S of the flip flop 97, and therefore, theflip flop 97 is set, and the control signal I shown in FIG. 8(i) whichbecomes H level when this arrival signal is supplied is obtained at theoutput Q. The control signal G which is the output Q of the flip flop 94shown in FIGS. 8(h) and (i) and control signal I which is the output Qof the flip flop 97 are supplied to the NOR gate 9l of the selectionsignal generating means 17.

Therefore, a control signal P shown in FIG. 8(j) is obtained at theoutput of the NOR gate 98. This control signal P is supplied to theexposure memory means 35 and exposure correction value calculation means31 in the imaging element control means 18 via the signal line 90. Forthe period during which this control signal P is at H level, these meansoperate and for the period during which this control signal P is at Llevel, the exposure time correction value of the exposure correctionvalue calculation means 31 as the output is held to a zero value and theexposure memory means 35 holds the last memory value (minimum value) atthe end of operation and the control of the imaging element controlmeans 18 is stopped.

That is, when the ALC area 120 is started, the exposure time becomes aminimum value (1 Tf). On the other hand, the control signal I becomes Hlevel again at this time point as described above, and therefore theiris control means 19 operates.

When operation starts and the brightness of the object is at someposition in the ALC area 120, there is a time point at which the valueof the above described error signal Yd becomes zero and an iris value Iyat that time point is stored and held in the iris value memory means 35.That is, for the three optimum set values at this time point, theexposure time is a minimum value (1 Tf), iris is Iy and AGC gain is aminimum value (0 dB) and the imaging element 6, iris 2 and amplifier 7operate with these values. While these values are held, the abovedescribed control signal I (see FIG. 8(i)) is held at H level. Thismeans that the iris control means 19 is operating for the periodcorresponding to the H level (the set value is determined at someposition in a section D of the time chart shown in FIG. 8).

Next, when the brightness of the object is very bright and the iris,that is, the aperture is narrowed to a maximum (when the aperturediameter is a minimum), the iris value remains Imin (the aperturediameter is a minimum) no matter how bright it may be. Reference numeral56 in the iris control means 19 in FIG. 5 denotes minimum iris valuejudging means which detects that this iris value has become Imin. Whenthe brightness of the object is very bright, control is performed suchthat the aperture diameter, that is, the iris value I is reduced.

Therefore, the iris value obtained from the second subtraction means 53in the iris control means 19 decreases and finally a time point appearsat which the iris value becomes Imin. This time point is detected by theminimum iris value judging means 56, the control signal obtained issupplied to the iris value memory means 55 and the iris value memorymeans 55 stores and holds this Imin.

As shown above, no matter in which area of ALC, STC, IRIS, AGC thebrightness (illuminance) of the object is located, if the image-takingmode is switched to a high-sensitivity image-taking mode and thatimage-taking mode is set, an optimum exposure time, iris value and AGCgain value which match the brightness are calculated, the values arestored in memory and held, and image taking is performed under anoptimum condition.

However, when image taking is realized in the high-sensitivityimage-taking mode and under an optimum condition, if the brightnessaround changes suddenly or when the place of image taking is changedfrom indoors to outdoors, or vice versa, the brightness of the objectchanges.

When the brightness around is brighter than the brightness with thethree optimum set values which are currently set in memory, if controlof the change from the above described dark state to bright state, thatis, a calculation of subtracting a correction value from the value onecycle ahead through the subtraction means of each control means in eachcontrol area is performed, the three optimum set values under thechanged and brighter condition are obtained.

On the contrary, if control of the change from the above describedbright state to dark state, that is, a calculation of adding acorrection value to the value one cycle ahead through the addition meansof each control means in each control area is performed, the threeoptimum set values under the changed and darker condition are obtained.

When the set value is stored in memory and the imaging element 6, iris 2and aplifier 7 are operated under that new image taking condition, it ispossible to realize optimum image taking in the new environment.

The control when this change from a bright condition to a dark conditiontakes place will be explained using drawings of an actual operation.

Suppose that the current state is at some position of the ALC area 120in FIG. 9 and an optimum condition is set in that condition. In such acase, the iris control means 19 is operating as described above, andtherefore a setting is made at some position in the section D in thetime chart shown in FIG. 8. If it becomes darker than the currentbrightness in this area, the iris value is small, and therefore thebrightness signal component value Y drops and becomes smaller than thereference value Ys of the brightness signal component. For example,suppose the value of the current iris value I is Ir and the value of Ybecomes Ys/4 when a dark situation is set in this condition, it ispossible to set the iris value to 4 Ir so as to make Y equal to Ys inthis dark condition. Setting a double value in terms of aperturediameter causes the incoming light quantity to become equal, andtherefore the value becomes a set value when it becomes dark. Asdescribed above, this set value can be obtained by calculating the iriscorrection value from the iris correction value calculation means 50 inthe iris control means 19 and preceding period iris value from the irisvalue memory means through the second addition means 54.

This is the control and optimum value setting method when it becomesdark in the area of ALC. Next, suppose the current state is at someposition in the ALC area 120 in FIG. 9 and an optimum condition is setin that situation. When the brightness is suddenly changed (darkened)from that condition to some position in the STC area 121. In this case,in the ALC area 120, since Y<Ys, the iris value I increases and finallyit reaches Ist (point a) on the boundary (dotted line 131) between theALC area 120 and STC area 121 shown in FIG. 9. Second iris value judgingmeans 59 in the iris control means 19 shown in FIG. 5 detects a timepoint at which the iris value reaches the above described Ist (point a).When the iris value obtained from the second addition means 54 reachesIst, the second iris value judging means 59 obtains an iris value Ist(point a) arrival signal shown in FIG. 8(e). This signal is supplied tothe OR gate 96 in (control means selection control signal generatingmeans) selection signal generating means 17 via a signal line 63. Thissignal passes through the OR gate 96 and is supplied to the reset inputR of the flip flop 97, and therefore the flip flop 97 is reset and thecontrol signal I shown in FIG. 8(i) is obtained at the output Q. Whenthe iris value Ist (point a) arrival signal shown in FIG. 8(e) issupplied to the selection signal generating means 17, the operation ofthe iris control means 19 is stopped (section D in FIG. 8 ends) and theiris value memory means 55 holds the above described iris value Ist. Onthe other hand, the control signal P shown in FIG. 8(j) is obtained fromthe NOR gate 98 of the selection signal generating means 17 and thissignal is supplied to the imaging element control means 18. Therefore,the STC area, that is, the control (section E in FIG. 8) by the imagingelement control means 18 starts from the time point at which the abovedescribed arrival signal is issued. Assuming that the brightness islocated at some position in the STC area, control is performed such thatthe exposure time m·Tf is increased until the exposure time that matchesthe brightness is obtained. As described above, when the control entersthe STC area 121, the exposure time starts from 1 Tf as shown in FIG. 9(position of dotted line 131). For example, if the brightness of thecurrent object is assumed to be at the position of the dotted line 146,the brightness signal component value Y at the position of the dottedline 131 becomes the value of Yb shown by reference numeral 148.Furthermore, reference numeral 147 denotes a reference value Yso of thebrightness signal component at this position and Yso=Yh. The errorsignal Yd at this position is Y=Yb<Ys =Yso=Yh, and therefore Yd(=Y−Ys)<0, and the exposure time correction value Δm·Tf in the precedingperiod obtained from the exposure correction value calculation means 31in the imaging element control means 18 for every M·Tf period passesthrough the terminal a (−) of the first switching means and is suppliedto the first addition means 34. The exposure time m·Tf in the precedingperiod from the exposure memory means 35 is also supplied to the firstaddition means 34. Therefore, exposure time m·Tf+Δm·Tf in the nextperiod, which is the sum of both values, is obtained. In this way, asthe control period is repeated several times, Y increase along a curve145 shown in FIG. 10. (Though this is shown continuously, Y increasesalong the curve periodically and discretely). Finally, when Y=Ys=Ysc,that is, when the error signal Yd approximates to zero, the firstjudging means 30 detects Yd=0 and the exposure memory means 35 storesand holds the value of exposure time at that time. In this way, a setvalue corresponding to the brightness of the object is determined andoptimum image taking can be realized.

This is the method for obtaining a set value that matches the newsituation when the brightness of the object which has been set to anoptimum set value in the ALC area 120 is switched to a dark situation ofSTC area 121.

Furthermore, a case where the brightness of the object is furtherchanged to a dark area situation will be explained.

Since the method of calculating an optimum set value in each area iseasily understandable from the explanations so far, only the method ofchanging from one area to another will be explained.

When the brightness of the object is suddenly changed from the ALC area120 to the IRIS area 122, control must be shifted from the ALC area 120through STC area 121 to the IRIS area 122. The method of shifting fromthe ALC area 120 to the STC area 121 has been described above. Thecontrol of the STC area 121 has also been explained.

In this case, since Y<Ys in the STC area 121, the exposure timeincreases and reaches a maximum value. A dotted line 140 in FIG. 9denotes the position of brightness of the maximum exposure time valueand indicates a boundary position at which the STC area 121 transitionsto the IRIS area 122.

Maximum exposure judging means 37 in the imaging element control means18 in FIG. 4 detects a time point at which the exposure timeperiodically obtained from the first addition means 34 reaches a maximumvalue (34 Tf) and generates a control signal at that time point. Anexposure time maximum value arrival signal shown in FIG. 8(f) isobtained from the maximum exposure judging means 37 and supplied to theOR gate 95 of the selection signal generating means 17 shown in FIG. 7via a signal line 43. This signal passes through the OR gate 95 and isadded to the set input S of the flip flop 97, and therefore a controlsignal I as shown in FIG. 8(i) is obtained at the output Q. As describedabove, this signal is added to the iris control means 19, and thereforethe control by the iris control means 19 starts from the time point atwhich the above described arrival signal is generated. The control of asection F in FIG. 8 is performed. The method of obtaining an optimum setvalue by the iris control means 19 is the same as that explained so far.

When the object is placed in the dark AGC area 123, the time point atwhich the iris value reaches a maximum value (position indicated by adotted line 141 in FIG. 9) is detected and the AGC gain control means 16may be operated from that time point. The maximum iris value judgingmeans 58 of the iris control means 19 in FIG. 5 detects the time pointat which the iris value obtained from the second addition means 54reaches a maximum value. An iris maximum value arrival signal shown inFIG. 8(g) is obtained from the maximum iris value judging means 58. Thissignal is supplied to the OR gate 93 of the selection signal generatingmeans 17 shown in FIG. 7 via the signal line 62. Since this signal ispassed through the OR gate 93 and added to the set input S of the flipflop 94, a control signal G shown in FIG. 8(h) is obtained at the outputQ. Since the control signal G is supplied to the AGC gain control means16 , this and subsequent areas become areas under the control of the AGCgain control means 16. The control of the AGC gain control means 16 iscontrol to obtain an optimum set value where Y=Ys=Y1 and the same asdescribed above.

Finally, in a completely dark situation, there is no brightness signalcomponent value Y, and therefore Y<Ys and the AGC gain value obtainedfrom the third addition means 72 in the AGC gain control means 16 inFIG. 6 increases up to a maximum value. When the AGC gain value reachesa maximum value, maximum gain judging means 77 generates a controlsignal of arrival of the maximum value at that time point. The maximumvalue arrival control signal from the maximum gain judging means 77 isadded to the AGC gain value memory means 75 and the AGC gain maximumvalue is stored and held. The set value in this condition is the same asthe initial set value at the control start point at which the mode isswitched to the high-sensitivity image-taking mode.

When this device is brought from this state to a bright state again, anoptimum set value is obtained if the control in the above describedbright direction is performed. In short, when this device is changedfrom the normal image-taking mode to the high-sensitivity image-takingmode, it is possible to obtain an optimum set value in any brightsituation, hold the set value and realize image taking. Moreover, whenthe device is moved from that setting condition to a situation withdifferent- brightness or when the place of image taking remains the sameand only the brightness changes, it is possible to automaticallytransition to a set value suited to the brightness.

The above described embodiments have shown the components with hardwareusing circuit blocks, but the imaging control means 25 in FIG. 1 mayalso be constructed of one microcomputer. In this case, the operationsof the respective blocks are expressed with a program. Furthermore,initial set values and brightness signal reference values or the likeare preset in an internal ROM.

When the present invention is used for not only a video camera but alsoa device furnished with imaging means such as a digital still camera, itis possible to obtain effects similar to those of the present invention.

1. An image processing device provided with a first image-taking mode used in a bright environment and a second image-taking mode used in a dark environment, comprising: a lens unit which forms an optical image of an object on an imaging element; an iris which adjusts a light quantity which has entered said lens unit; an imaging element having an electronic shutter function of outputting the optical image of the object for which the light quantity from said iris is adjusted as an image signal; an AGC amplifier which amplifies an image/video signal from said imaging element and can adjust an amplification gain thereof; signal processing means for obtaining a video signal by subjecting the image signal amplified by said AGC amplifier to signal processing; comparison means for comparing the brightness signal level of said video signal indicating the brightness of the object with a predetermined reference brightness signal level; and imaging control means, wherein in said second image-taking mode, said imaging control means changes the length of period of said electronic shutter function for every period of a multiple of two fields, continuously changes the electronic shutter-ON time (exposure time) in accordance with the period and holds the electronic shutter-ON time at a time point at which the output of said comparison means at which said brightness signal level matches said reference brightness signal level becomes 0 (zero).
 2. The image processing device according to claim 1, wherein the imaging control means comprises iris control means for adjusting said iris when the brightness around is brighter than a predetermined value and darker than a predetermined value and holding the iris when the output of the comparison means at which the brightness signal level matches the reference brightness signal level becomes 0 (zero).
 3. The image processing device according to claim 1, wherein the imaging control means comprises gain control means for adjusting the gain of the AGC amplifier when the brightness around is darker than a predetermined value and holding the gain value when the output of said comparison means at which said brightness signal level matches said reference brightness signal level becomes 0 (zero). 4-11. (canceled) 